It looks like this can be done by passing an extra argument to busybox's udhcpc, but I can't find where Tomato actually *calls* udhcpc or whether it's possible to stuff a value into an environment variable (NVRAM or otherwise) that it will pick up.

I'd advocate allowing passing of an arbitrary string (via NVRAM?) to udhcpc rather than implementing individual parameters like vendor class, which would allow for quite a bit of flexibility and future-proofing with very little code change. =) Just my $0.02.